Several polypeptides present in the thymus gland have been implicated to play an important role in the development and maintenance of immunological competence in animals, including man. The importance of the immune system in the defense against cancer and tumor cells is now widely recognized. In recent years, a few polypeptides shown to be able to stimulate maturation, differentiation and function of T cells have been isolated from calf thymus.
Particularly a heat-stable fraction has been prepared from calf thymus extracts and designated Thymosin Fraction 5. Studies have shown that Thymosin Fraction 5 is a potent immunopotentiating preparation and that it can act in lieu of the thymus gland to reconstitute immune functions in thymic deprived or immunodeprived individuals. Peptides which have been isolated from Thymosin Fraction 5 include thymosin .alpha..sub.1, an acidic peptide containing 28 amino acid residues, the structure of which has been described in U.S. Pat. No. 4,079,127, and thymosin .beta..sub.4 containing 44 amino acid residues [Low, T.L.K., et al., Proc. Nat. Acad. Sci. 78, 1162-1166 (1981)],